Inclusion of nitrogen at the silicon dioxide-silicon carbide interface for passivation of interface defects

ABSTRACT

In one aspect the present invention provides a method for manufacturing a silicon carbide semiconductor device. A layer of silicon dioxide is formed on a silicon carbide substrate and nitrogen is incorporated at the silicon dioxide/silicon carbide interface. In one embodiment, nitrogen is incorporated by annealing the semiconductor device in nitric oxide or nitrous oxide. In another embodiment, nitrogen is incorporated by annealing the semiconductor device in ammonia. 
     In another aspect, the present invention provides a silicon carbide semiconductor device that has a 4H-silicon carbide substrate, a layer of silicon dioxide disposed on the 4H-silicon carbide substrate and a region of substantial nitrogen concentration at the silicon dioxide/silicon carbide interface.

This application claims the benefit under 35 U.S.C. § 119 (e) of U.S. Provisional Application No. 60/192,009, filed Mar. 24, 2000, which is incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under grant # MDA972 98-1-0007 awarded by the Defense Advanced Research Projects Agency (DARPA). The federal government has certain rights in the present invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to silicon carbide semiconductor devices and methods for their construction. More specifically, the instant invention relates to silicon carbide semiconductor devices with a layer of silicon dioxide disposed on a silicon carbide substrate that have a region of substantial nitrogen concentration at the silicon dioxide-silicon carbide interface.

2. Description of the Related Art

Presently, the silicon (Si) metal-oxide-semiconductor field effect transistor (MOSFET) is the most commonly manufactured transistor in the world. Silicon carbide (SiC) MOSFETS are currently being developed because of superior material properties that provide high power generation, conditioning, transmission and the ability to operate at higher temperatures.

Like Si, SiC thermally oxidizes to form SiO₂. However, the oxidation kinetics and oxide interface of SiC are poorly understood in comparison to Si. The oxidation rate of SiC is lower than Si by more than a factor of 10. Furthermore, the quality of the SiO₂/SiC interface is also inferior to the SiO₂/Si interface. This problem causes low channel mobilities in SiC MOSFETS due to the density of interface states that exists in the band gap near the conduction band edge. Refinements in cleaning and oxidation techniques have improved the interface quality of SiO₂/SiC, but producing reliable devices will require additional improvements.

Oxynitride films on Si have superior device reliability, better dopant diffusion resistance and higher dielectric quality than pure oxide films. Methods for manufacturing oxynitride films on Si include annealing SiO₂/Si in N₂O or NO, with NO believed to be the species responsible for nitrogen incorporation. Nitrogen is typically incorporated near the SiO₂/Si interface at concentrations of about 10¹⁵ cm⁻², although the actual profile is a complicated function of the gaseous species and the temperature cycle. The atomic process for nitrogen incorporation appears to involve NO interstitial diffusion through the oxide to the SiO₂/Si interface where a reaction takes place that forms an oxynitride film.

Initial studies of oxynitride films on 6H—SiC have shown a small improvement in MOS interface quality after NO anneal (reduction of the interface state and fixed charge densities takes place) but inferior MOS interface quality after N₂O anneal. These results are surprising considering that nitrogen incorporation at the SiO₂/Si interface does not decrease the interface trap density. Secondary ion mass spectrometry (SIMS), reveals that NO anneal incorporates nitrogen near the SiO₂/4H—SiC interface.

Thus, there exists a need for a method that reduces interface trap density near the conduction band edge at the SiO₂/SiC interface. By increasing channel mobility, such a method would facilitate the manufacture of high quality SiC MOSFET devices.

SUMMARY OF THE INVENTION

The present invention addresses this need by providing a method that reduces interface trap density in silicon carbide MOS devices and a semiconductor device with reduced interface trap density. This is accomplished by incorporating nitrogen at the SiO₂/SiC interface.

In one aspect, the present invention provides a method for manufacturing a semiconductor device. A layer of silicon dioxide is formed on a silicon carbide substrate. Then, nitrogen is incorporated at the silicon dioxide/silicon carbide interface. In one embodiment, the silicon carbide substrate is 4H—SiC. In another embodiment, the silicon carbide substrate is 6H—SiC. In yet another embodiment, the silicon carbide substrate is doped with a p-type or n-type dopant.

The silicon dioxide layer is formed by cleaning and then oxidizing the silicon carbide substrate. Preferably, the silicon carbide substrate is cleaned with 10% HF and then thermally oxidized. In a preferred embodiment, the silicon carbide substrate is oxidized at between about 900° C. and about 1200° C. Preferably, the silicon dioxide layer is greater than about 10 Å thick.

In one embodiment, nitrogen is incorporated by annealing the semiconductor device in nitric oxide or nitrous oxide. Preferably, in this embodiment, the semiconductor device is annealed at between about 950° C. and about 1200° C. (more preferably, at about 1175° C.) for between about 1 and about 4 hours. In another embodiment, nitrogen is incorporated by annealing the semiconductor device in ammonia at about between 950° C. and about 1200° C. (more preferably, at about 1175° C.) for about 4 hours.

Preferably, the areal density of nitrogen at the silicon dioxide/silicon carbide interface is between about 0.5×10¹⁴ cm⁻² and about 1×10¹⁶ cm⁻² (more preferably, between about 1×10¹⁴ cm⁻² and about 2×10¹⁵ cm⁻²). In a preferred embodiment, the maximum concentration of nitrogen at the silicon dioxide/silicon carbide interface is about 0.5%.

In another aspect, the present invention provides a silicon carbide semiconductor device. The semiconductor device has a silicon carbide substrate with a layer of silicon dioxide disposed thereon. The silicon dioxide/silicon carbide interface is characterized by a region of substantial nitrogen concentration.

In one embodiment, the silicon carbide substrate is 4H—SiC. In another embodiment, the silicon carbide substrate is 6H—SiC. In yet another embodiment, the silicon carbide substrate is doped with a p-type or a n-type dopant. Preferably, the areal density of nitrogen at the silicon dioxide/silicon carbide interface is between about 0.5×10¹⁴ cm⁻² and about 1×10¹⁶ cm⁻² (more preferably, between about 1×10¹⁴ cm⁻² and about 2×10^(15 cm) ⁻²).

In one preferred embodiment, the maximum concentration of nitrogen at the silicon dioxide/silicon carbide interface is about 0.5%. In another preferred embodiment, the layer of silicon dioxide is greater than about 10 Å thick.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a SiO₂/SiC semiconductor device of the current invention.

FIG. 2 is an excitation curve of the nuclear reaction of ¹⁵N(p,αγ) ¹²C (E_(R)=429 keV) for SiO₂/SiC annealed in 10 mbar of ¹⁵N¹⁸O at 1000° C. for 4 h. The dots are the experimental data and the lines are the simulated excitation curves, components and sum, assuming the ¹⁵N profiles shown in FIG. 4.

FIG. 3 is an excitation curve of the nuclear reaction of ¹⁸O(p,α)¹⁵N (E_(R)=151 keV) for SiO₂/SiC annealed in 10 mbar of ¹⁵N¹⁸ O at 1000° C. for 4 h. The dots are the experimental data and the lines are the simulated excitation curves, components and sum, assuming the ¹⁸ O profiles shown in FIG. 6.

FIG. 4 is the concentration profile of ¹⁵N in SiO₂/SiC annealed in 10 mbar of ¹⁵N¹⁸O at 1000° C. for 1 h (dotted) and 4 h (solid). The origin corresponds to the surface and the SiO₂/SiC interface is at about 30 nm.

FIG. 5 is the concentration profile of ¹⁵N in SiO₂/Si annealed in 10 mbar of ¹⁵N¹⁸O at 1000° C. for 1 h (dotted) and 4 h (solid). The origin corresponds to the surface and the SiO₂/Si interface is at about 20 nm.

FIG. 6 is a concentration profile of ¹⁸O in SiO₂/SiC annealed in 10 mbar of ¹⁵N ⁸O at 1000° C. for 1 h (dotted) and 4 h (solid). The origin corresponds to the surface and the ¹⁸O gradient is due to the surface exchange reaction.

FIG. 7 is a concentration profile of ¹⁸O in SiO₂/Si annealed in 10 mbar of ¹⁵N¹⁸O at 1000° C. for 1 h (dotted) and 4 h (solid). The origin corresponds to the surface and the ¹⁸O gradient is due to the surface exchange reaction.

FIG. 8 illustrates the electrical effects of passivation techniques for SiO₂/6H—SiC and SiO₂/4H—SiC samples annealed in NO for about two hours at about 1100° C. Similar results are achieved with NH₃.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to preferred embodiments of the invention. While the invention will be described in conjunction with the preferred embodiments, it will be understood that it is not intended to limit the invention to those preferred embodiments. To the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

FIG. 1 schematically illustrates a semiconductor device of the current invention. Semiconductor device 112 has a silicon dioxide layer 110 disposed on silicon carbide substrate 102. Silicon dioxide layer 110 consists of two different regions 106 and 108. Region 108, which is located at the SiO₂/SiC interface 104, is a region of substantial nitrogen concentration. The nitrogen concentration of region 106 is substantially lower than the nitrogen concentration of region 108.

Preferably, silicon dioxide layer 110 is greater than about 200 Å thick. Preferably, region 106 is between about 200 Å thick and about 1000 Å thick (more preferably, between about 300 Å thick and about 500 Å thick) Preferably, region 108 is between about 10 Å thick and about 30 Å thick (more preferably, between about 20 Å thick and about 30 Å thick).

The silicon carbide substrate 102 is 4H—SiC or 6H—SiC (preferably, 4H—SiC) and may be doped with either a p-type or a n-type dopant. The silicon carbide substrate 102 may also include an epitaxial layer. Preferably, the areal density of nitrogen at the silicon dioxide/silicon carbide interface 104 is between about 0.5×10¹⁴ cm⁻² and about 1×10¹⁶ cm⁻² (more preferably, between about 1×10¹⁴ cm⁻² and about 2×10¹⁵ cm⁻²). The nitrogen concentration at the silicon dioxide/silicon carbide interface is between about 10²²/cm³ and about 10²⁰/cm³. The maximum concentration of nitrogen at the silicon dioxide/silicon carbide interface is about 0.5%.

The semiconductor device shown in FIG. 1 may be made by methods well known to those of ordinary skill in the art. For example, silicon dioxide may be deposited on a silicon carbide substrate by conventional deposition methods. The device may then be annealed with a nitrogen containing gas to provide a region of high nitrogen concentration such as area 108 at the SiO₂/SiC interface 104.

The silicon carbide substrate is 4H—SiC or 6H—SiC (preferably, 4H—SiC) and may be doped with either a p-type or a n-type dopant. The layer of silicon dioxide is preferably formed by cleaning the silicon carbide substrate followed by oxidation of the silicon carbide substrate. Preferred cleaning procedures include standard organic, heavy metals and RCA cleaning methods. Preferably, the silicon carbide substrate is cleaned with 10% HF. In a preferred embodiment, the silicon carbide substrate is thermally oxidized using standard wet or dry techniques. Preferably, the silicon carbide substrate is oxidized at between about 900° C. and about 1200° C. (more preferably, between about 1050° C. and about 1200° C.).

Nitrogen may be incorporated at the silicon carbide silicon dioxide interface by annealing in the presence of a nitrogen containing gas. Preferably, the nitrogen containing gas is nitric oxide, nitrous oxide or ammonia. When the nitrogen containing gas is nitric oxide, nitrous oxide or ammonia, the semiconductor device is annealed at between about 950° C. and about 1200° C. (more preferably, between about 1050° C. and about 1200° C., (most preferably, at about 1175° C.)) for between about 1 and about 4 hours.

Preferably, the areal density of nitrogen at the silicon dioxide/silicon carbide interface 104 is between about 0.5×10¹⁴ cm⁻² and about 1×10¹⁶ cm⁻² (more preferably, between about 1×10¹⁴ cm⁻² and about 2×10¹⁵ cm⁻²). The nitrogen concentration at the silicon dioxide/silicon carbide interface is between about 10²⁰/cm³ and about 10²²/cm³. The maximum concentration of nitrogen at the silicon dioxide/silicon carbide interface is about 0.5%.

EXAMPLES

The invention having been described, the following examples are presented to illustrate, rather than limit, the scope of the invention. The examples illustrate various embodiments and features of the present invention.

Example 1 Preparation of Annealed 4H—SiC Wafers

Preparation of silicon-faced (8° off axis) 4H—SiC wafers with a p-type substrate and epitaxial layer, doped at about 2×10¹⁶ cm⁻³ and n-type (100) Si wafers with a resistivity of about 10 Ω-cm required etching in a 10% HF solution to remove native oxide and thermal oxidation in a resistively heated quartz tube furnace. The 4H—SiC wafers were oxidized in wet oxygen (oxygen bubbled through deionized water at about 85-90° C.) at a flow rate of about 1.0 l/mm. The 4H—SiC wafers were then loaded in a wet oxidizing ambient at about 850° C., the temperature was raised to about 1150° C. and the samples were oxidized for about 30 minutes in accordance with prior art techniques. The ambient was then switched to argon, the temperature was lowered to about 800° C. and the SiC wafers were removed from the furnace. The oxide thickness for the SiC wafers was about 30 nm.

When annealing was carried out with ¹⁵N containing gases the following conditions were typically employed. The 4H—SiC wafers were annealed in a resistively heated quartz tube furnace in 99.7% ¹⁵N¹⁸O or ¹⁵N₂O at a static pressure of about 10 mbar at about 1000° C. for either about 1 or about 4 hours. Alternatively, the 4H—SiC wafers were annealed in 99.7% ¹⁵NH₃ at a static pressure of about 10 mbar at about 1100° C. for about 4 hours.

However, when annealing was carried out with ¹⁴N containing gases different conditions were typically employed. Annealing was preferably carried out at gas flow rates of about 0.5 l/min at atmospheric pressure and at temperatures between about 1000° C. and about 1200° C. for between about one and about six hours.

Example 2 Preparation of Annealed 4H—Si Wafers

Si wafers were oxidized in dry oxygen (<2 ppm H₂O) at a flow rate of about 1 l/min, loaded in a oxidizing ambient at about 900° C. and oxidized at about 900° C. for about 25 minutes. The ambient was then switched to argon, and the Si wafers were removed from the furnace. The oxide thickness for Si wafers was about 20 nm.

The Si wafers were then annealed in a resistively heated quartz tube furnace with 99.7% ¹⁵N¹⁸O or ¹⁵N₂O at a static pressure of about 10 mbar at about 1000° C. for either about 1 or about 4 hours. When annealing was carried out with ¹⁴N containing gases the conditions specified above were typically used. Anneals were preferably carried out at gas flow rates of about 0.5 l/min at atmospheric pressure, at temperatures between about 1000° C. and about 1200° C. for between about one and about six hours.

Example 3 Measurement of Areal Densities

The areal densities of ¹⁵N and ¹⁸O of annealed Si/C and Si wafers prepared as described in Examples 1 and 2 were determined by Nuclear Reaction Analysis (NRA) using the reactions ¹⁵N(p,αγ) ¹²C at E_(p)=1 MeV and ¹⁸O(p,αγ)¹⁵N at E_(p)=730 keV. The ¹⁵N and ¹⁸O depth profiles were determined by nuclear resonance profiling (NRP) using the resonances in the reactions ¹⁵N(p,αγ)¹²C at 429 keV and ¹⁸O(p,α)¹⁵N at 151 keV, respectively, and a tilted sample geometry (ψ=65°). The measured excitation curves (γ or α yield versus incident proton energy) around the resonance energy were converted into concentration versus depth profiles. Table 1, shown below summarizes oxidation conditions, anneal times, oxide thickness and areal densities for a number of experiments.

TABLE 1 ¹⁵N ¹⁸O Oxide Anneal ¹⁵N Areal ¹⁸O Areal Thick- Oxidation Time Density Density ness Material Conditions (hours) (10¹⁵ cm⁻²) (10¹⁵ cm⁻²) (nm) Si 900° C.-Dry 1 1.6 19.6 20.3 Si 900° C.-Dry 4 2.2 44.6 22.8 SiC 1150° C.-Wet 1 0.1 15.9 28.9 SiC 1150° C.-Wet 4 0.2 51.0 27.6

An excitation curve and its simulation for the ¹⁵N(p,αγ)¹²C reaction in a SiO₂/SiC sample annealed in ¹⁵N¹⁸O are shown in FIG. 2. The obtained depth profiles of ¹⁵N in the SiO₂/SiC samples (FIG. 4) are similar to the profiles in the SiO₂/Si samples (FIG. 5). Nitrogen is concentrated primarily at the SiO₂/SiC interface at about 30 nm. The width of the N profile is larger in the SiO₂/SiC samples than in the SiO₂/Si samples, which suggests that the SiO₂/SiC interface is rougher than the SiO₂/Si interface. These depth profiles, determined by nuclear resonance profiling, provide independent verification of SIMS results, which showed nitrogen located near the interface.

The ¹⁵N areal density in SiO₂/SiC wafers is about 10¹⁴ cm⁻² for both anneal times (see Table 1), and the maximum nitrogen concentration is about 0.5% (see FIG. 4) when the wafers are annealed in ¹⁵N¹⁸O. Both of these values are much smaller than SiO₂/Si wafers, where the ¹⁵N areal density is about 10¹⁵ cm⁻² and the maximum concentration is about 10%. The rate of nitrogen incorporation in both SiO₂/SiC and SiO₂/Si wafers is related to their relative oxidation rates, which suggests that the rate of Si—N bonding at the interface is governed by the same factors as the oxidation rate in each material.

An excitation curve and its simulation for the ¹⁸O(p,α)¹⁵N reaction in a SiO₂/SiC wafer annealed in ¹⁵N¹⁸O are shown in FIG. 3. The ¹⁸O depth distributions in the SiO₂/SiC wafers (FIG. 6) show a gradient of ¹⁸O in the oxide from the surface to the interface. These ¹⁸O profiles, as well as the ¹⁸O areal densities (see Table 1), strongly resemble those of the Si wafers (FIG. 7). In Si wafers however, ¹⁸O accumulates at the interface, while little or no new oxidation occurs in SiC wafers. These results confirm the difference in oxidation rates of SiC and Si and indicate that the surface exchange mechanism is identical for oxides on SiC and Si. Since the exchange reaction is solely a characteristic of the oxide and not the substrate, this was further evidence of similar chemical behavior of oxide on SiC and Si substrates.

In SiO₂/SiC wafers annealed in ¹⁵N₂O, the amount of nitrogen incorporated is about 10^(13 cm) ⁻², which is near the detection limit of NRA and prevents resonance profiling. The amount of incorporated nitrogen is again an order of magnitude smaller than the amount of nitrogen in the SiO₂/Si samples, which is about 10¹⁴ cm⁻².

The above results are particularly meaningful when the electrical properties of oxynitrides on SiC are considered. Previous studies showed that thermal annealing of SiO₂/6H—SiC in NO at 1100° C. modestly decreased the interface trap density (D_(it)), while N₂O anneal actually increased D_(it). In contrast, the present invention demonstrates that rapid thermal annealing of SiO₂/4H—SiC in NO at 1100° C. significantly decreases the interface trap density (D_(it)).

A small but definite incorporation of nitrogen near the SiO₂/SiC interface from NO anneal and considerably less nitrogen incorporation from N₂O anneal is observed (see FIGS. 2-7). Thermal decomposition of NO at temperatures >1000° K. provides a fairly constant concentration of NO along with small amounts of N₂ and O₂. Conversely, N₂O readily decomposes into N₂ (about 60%), O₂ (about 25%), and NO (about 15%). The dominant reaction when oxide films are annealed in N₂O is dry oxidation of the substrate. Annealing in N₂O incorporates very little nitrogen at the interface because of the low NO concentration and leads to poor quality, dry oxidation of SiC, which provides a high interface trap density. The re-oxidation technique (annealing SiO₂/SiC, in wet oxygen at 950° C.) improves interface quality without appreciable new oxide growth by reducing D_(it) near the middle of the band gap. Nitrogen incorporation at the SiO₂/4H—SiC interface may offer benefits similar to the re-oxidation anneal, by substantially reducing D_(it) near the conduction band edge, which also could explain the different results in NO and N₂O anneals.

Thus, annealing SiO₂/SiC wafers in N₂O incorporates less nitrogen (about 10¹³ cm⁻²) while annealing in NO incorporates nitrogen near the SiO₂/SiC interface with an areal density of about 10¹⁴ cm⁻² and an atomic concentration of about 0.5%. Annealing in NH₃ provides reductions in D_(it) similar to those that are achieved using NO; however, NH₃ annealing incorporates nitrogen in the SiO₂ layer with a much higher areal density (about 10¹⁶ cm⁻²).

Compared to SiO₂/Si, nitrogen incorporation is lower for SiO₂/4H—SiC annealed in NO but higher for SiO₂/4H—SiC annealed in NH₃ These values are an order of magnitude lower than comparably prepared SiO₂/Si wafers. Like SiO₂/Si, a surface exchange of oxygen in the gas phase with the oxide followed by NO diffusion and reaction at the SiO₂/SiC interface are observed after NO anneal. The surface exchange reaction for SiO₂/SiC appears to be similar to SiO₂/Si confirming that the oxides themselves are chemically similar. The incorporation of nitrogen at the interface provides a possible explanation for the electrical characteristics of NO, NH₃ and N₂O annealed SiO₂/4H—SiC wafers. In particular, the interface trap density near the conduction band in SiO₂/4H—SiC wafers is substantially reduced after high temperature annealing in either NO and NH₃. These results suggest that any process that incorporates atomic nitrogen at the SiO₂/SiC interface may reduce the interface trap density in SiO₂/4H—SiC wafers. The process described above facilitates improvements in the channel mobility for inversion mode, n-channel MOSFETS, fabricated with 4H—SiC.

Example 4 Comparison of Annealing of SiO₂/6H—SiC and SiO₂/4H—SiC Wafers

FIG. 8 illustrates the effect of annealing SiO₂/6H—SiC and SiO₂/4H—SiC samples in nitric oxide (NO) and hence serves to show the electrical effects of the passivation techniques of the current invention. Similar results are observed using ammonia (NH₃) instead of NO. A reduction in the interface state density near the conduction band edge of approximately one order of magnitude is observed for the SiO₂/4H—SiC sample compared to the SiO₂/6H—SiC sample. This experimental observation is in agreement with prior art suggestions that indicate a large interface state density exists for all polytypes of SiC at around 2.9 eV above the valence band edge. In 6H—SiC, these states lie mostly in the conduction band and hence are not affected by passivation techniques. However, 4H—SiC, the material of choice for SiC MOSFET fabrication, has a wider band gap than 6H—SiC (˜3.3 eV compared to ˜3 eV), and as a result, a much larger fraction of the interface states near 2.9 eV lie in the band gap of 4H—SiC where they act to degrade channel mobility for n-channel, inversion mode devices through field termination, carrier (electron) trapping and Coulomb scattering. Therefore, the current invention, as shown in FIG. 8, by identifying passivation techniques for these states, significantly improves prospects for SiC MOSFET manufacture.

Finally, it should be noted that there are alternative ways of implementing both the process and apparatus of the present invention. For example, the silicon carbide semiconductor substrate may be annealed at different temperature, pressures or even with different nitrogen containing gases. The silicon carbide semiconductor device may contain regions of higher or lower nitrogen concentration at the silicon carbide/silicon dioxide interface than described in the present invention. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims. 

1-19. (canceled)
 20. The semiconductor device of claim 22, wherein the silicon carbide substrate is 4H—SiC.
 21. The semiconductor device of claim 22, wherein the silicon carbide substrate is 6H—SiC.
 22. A semiconductor device comprising: a silicon carbide substrate; a layer of silicon dioxide disposed on the silicon carbide substrate to create a silicon dioxide/silicon carbide interface with an interface trap density; and a region of substantial nitrogen concentration at the silicon dioxide/silicon carbide interface for reduction in the interface trap density, wherein the silicon carbide substrate is doped with a p-type or a n-type dopant, and wherein the nitrogen is incorporated by annealing the semiconductor device in ammonia. 23-26. (canceled)
 27. A semiconductor device comprising: a silicon carbide substrate; a layer of silicon dioxide disposed on the silicon carbide substrate to create a silicon dioxide/silicon carbide interface with an interface trap density; and a region of substantial nitrogen concentration at the silicon dioxide/silicon carbide interface for reduction in the interface trap density, wherein the nitrogen is incorporated by annealing the semiconductor device in ammonia.
 28. The semiconductor device of claim 27, wherein the silicon carbide substrate is doped with a p-type or a n-type dopant.
 29. The semiconductor device of claim 27, wherein the silicon carbide substrate is 4H—SiC.
 30. The semiconductor device of claim 29, wherein the silicon carbide substrate is 6H—SiC.
 31. The semiconductor device of claim 27, wherein the layer of silicon dioxide is greater than about 10 Å thick.
 32. The semiconductor device of claim 27, wherein the semiconductor device is annealed at about between 950° C. and about 1200° C. for about 4 hours.
 33. The semiconductor device of claim 32, wherein the semiconductor device is annealed at about between 1175° C.
 34. The semiconductor device of claim 27, wherein an areal density of nitrogen at the silicon dioxide/silicon carbide interface is in the range of about 0.5×10^(14 cm) ⁻² and about 1×10¹⁶ cm⁻².
 35. The semiconductor device of claim 34, wherein the areal density of nitrogen at the silicon dioxide/silicon carbide interface is in the range of about 1×10¹⁴ cm⁻² and about 2×10¹⁵ cm⁻².
 36. The semiconductor device of claim 27, wherein the maximum concentration of nitrogen at the silicon dioxide/silicon carbide interface is about 0.5%. 